In a wireless communication apparatus, a mixer circuit is typically used to carry out frequency conversion from an IF (Intermediate Frequency) signal for signal processing, which uses a relatively low frequency, into an RF (Radio Frequency) signal for communication, which uses a relatively high frequency, or frequency conversion from the RF signal into the IF signal.
FIG. 1 is a circuit diagram showing the configuration of a single-balanced mixer circuit used in a wireless communication apparatus and the like.
As shown in FIG. 1, the single-balanced mixer circuit includes two mixer elements 51 and 180-degree phase combination circuit 52. Mixer elements 51 mix two opposite-phase IF signals (differential signals) with two in-phase local oscillation signals (hereinafter referred to as LO signals), and outputs an upper sideband signal and a lower sideband signal necessary for communication.
In 180-degree phase combination circuit 52, two input signals are combined with a 180-degree phase difference therebetween and the combined signal is outputted. Therefore, the upper sideband signal and the lower sideband signal, which mixer elements 51 have produced from the two differential IF signals, undergo in-phase combination in 180-degree phase combination circuit 52, and a resultant RF signal used in communication is outputted. Mixer elements 51 also output LO signals that are unnecessary for communication. The two in-phase LO signals inputted to mixer elements 51 are outputted as in-phase signals, the output signals undergo opposite-phase combination in 180-degree phase combination circuit 52, so that they are cancelled and removed.
Further, 180-degree phase combination circuit 52 shown in FIG. 1 can also be used as a 180-degree phase splitter by inputting a signal to the output port (Output) and taking out signals from the input ports (0, 180). In this case, by inputting RF signals and LO signals to the mixer elements, two IF signals in which the phase of one of which differs from the other by 180 degrees, can be obtained. Such a circuit that splits or combines signals with a 180-degree phase difference therebetween is used in various applications, such as a circuit that converts differential signals into non-differential signals or converts non-differential signals into differential signals, a circuit that splits differential signals to a plurality of active elements, and a circuit that combines differential signals. Therefore, there has recently been an increasing demand to use the 180-degree phase combination circuit (180-degree phase splitter) in microwave ICs used in a wireless communication apparatus and the like. In a microwave IC, a CPW (Coplanar Waveguide) line has been widely used as a transmission line because no backside processing of the substrate is required.
To split high-frequency signals and impart a 180-degree phase difference to the two split signals, a rat race circuit is typically used. A rat race circuit is a circuit in which a signal line is split into two to split signals in such a way that the split two signal lines are different in length by half the wavelength of the frequency of the signal to be transmitted so as to impart a 180-degree phase difference to the two split signals.
However, the line length corresponding to half the wavelength of the signal frequency ranges from several millimeters to several centimeters even in the case of a high-frequency signal on the order of GHz or higher, and such a length requires a large circuit footprint. It is therefore difficult to incorporate a rat race circuit in a microwave IC.
To address the above problem, instead of using the difference in line length to provide a phase difference, there is a method for providing a 180-degree phase difference by using a balun circuit that converts a non-differential transmission line, such as the CPW line described above and a microstrip line, into a differential transmission line, such as a slot line and a CPS (Coplanar Strips) line, or a balun circuit that converts a differential transmission line into a non-differential transmission line. Such a method is proposed in non-patent document 1 (Mu-Jung Hsieh, Chun-Yi Wu, Chi-Yang Chang, and Dow-Chin Niu, “Broadband mm-wave Schottky diode frequency doubler using a broadband CPW balun”, The 6th topical symposium on millimeter waves (TSMMW 2004) technical digest, pp. 285-288, February 2004).
As shown in FIG. 2, the balun circuit described in non-patent document 1 includes first FCPW (Finite Ground Coplanar Waveguide) line 61, second FCPW line 62a, and third FCPW line 62b, which serve as signal input/output ports; first CPS line 64a and second CPS line 64b, which are differential transmission lines; FCPW-CPS converter/splitter 65 that converts first FCPW line 61 into first CPS line 64a and second CPS line 64b; first CPS-FCPW converter 67a that converts first CPS line 64a into second FCPW line 62a; and second CPS-FCPW converter 67b that converts second CPS line 64b into third FCPW line 62b, all of which are formed on substrate 69.
First FCPW line 61, second FCPW line 62a, and third FCPW line 62b are non-differential transmission lines, each including a center conductor and two grounded conductors disposed in such a way that they sandwich the center conductor. The grounded conductors, two in each of first FCPW line 61, second FCPW line 62a, and third FCPW line 62b, are connected to each other via air bridge 68.
In the balun circuit shown in FIG. 2, FCPW-CPS converter/splitter 65 splits and converts first FCPW line 61 into first CPS line 64a and second CPS line 64b. First CPS-FCPW converter 67a converts first CPS line 64a into second FCPW line 62a, and second CPS-FCPW converter 67b converts second CPS line 64b into third FCPW line 62b. The center conductor of second FCPW line 62a is connected to the center conductor of first FCPW line 61, and the center conductor of third FCPW line 62b is connected to the grounded conductor of first FCPW line 61. The grounded conductor of second FCPW line 62a is connected to the grounded conductor of first FCPW line 61, and the grounded conductor of third FCPW line 62b is connected to the center conductor of first FCPW line 61.
By thus reversing the connection of the center conductor and the grounded conductor of second FCPW line 62a with the center conductor and the grounded conductor of first FCPW line 61 with respect to the connection of the center conductor and the grounded conductor of third FCPW line 62b with the center conductor and the grounded conductor of first FCPW line 61, a signal inputted to first FCPW line 61 becomes differential signals in which the phase of one differs from the other by 180 degrees. The differential signals are outputted from second FCPW line 62a and third FCPW line 62b. 
In non-patent document 1, the length of each of first CPS line 64a and second CPS line 64b coincides with one-fourth the wavelength of the signal frequency. However, since the balun circuit shown in FIG. 2 does not provide a phase difference based on the line length unlike a rat race circuit, the length of each of first CPS line 64a and second CPS line 64b does not necessarily coincide with one-fourth the wavelength of the signal frequency.
In the balun circuit described in non-patent document 1 described above, since the grounded conductors of the first FCPW line, the second FCPW line, and the third FCPW line are not interconnected, the potentials thereof will not always be the same. In non-patent document 1, a frequency multiplier that multiplies the frequency of an input signal is configured by using the balun circuit shown in FIG. 2 as a 180-degree phase splitter, connecting a diode, which is a two-terminal element, to each of the second FCPW line and the third FCPW line, and combining the outputs of the diodes. In such a circuit configuration, different potentials of the grounded conductors of the FCPW lines will not particularly be a problem.
However, when a single-balanced mixer circuit shown in FIG. 3 is configured by using the balun circuit, for example, shown in FIG. 2 as a 180-degree phase combination circuit and by connecting a three-terminal active element, such as an FET, used as a mixer element, to each of the second FCPW line and the third FCPW line, the following problem will occur.
In the single-balanced mixer circuit shown in FIG. 3, the source electrode of one of FETs 71a is connected to the grounded conductor of the second FCPW line, and the source electrode of the other FET 71b is connected to the grounded conductor of the third FCPW line. Each of the gate electrodes of two FETs 71a and 71b is connected to an LO signal source and a bias (Vg) source. The center conductor of the second FCPW line is connected to the drain electrode of FET 71a via capacitor 72a, and the center conductor of the third FCPW line is connected to the drain electrode of FET 71b via capacitor 72b. 
The drain electrode of FET 71a is connected not only to capacitor 73a, the other end of which is connected to the grounded conductor, but also to a stub having a predetermined length, and an (opposite phase) IF signal is supplied through the stub. Similarly, the drain electrode of FET 71b is connected not only to capacitor 73b, the other end of which is connected to the grounded conductor, but also to a stub having a predetermined length, and an IF signal is supplied through the stub The capacitance (impedance) of each of s capacitors 73a and 73b is open when viewed from the drain electrode side at the frequency of the RF signal, and is set to a value at which the insertion loss is minimized at the frequency of the IF signal.
In such a configuration, an upper sideband signal, a lower sideband signal, and LO signals are outputted from the drain electrodes of FETs 71a and 71b, each of which is a mixer element, The upper sideband signal and the lower sideband signal undergo in-phase combination in the balun circuit, and the LO signals undergo opposite-phase combination in the balun circuit.
However, in the configuration shown in FIG. 3, the source electrodes of two FETs 71a and 71b, which should normally be grounded have different potentials at connection sections 74a and 74b, so that the operation conditions of FETs 71a and 71b are disadvantageously different from each other.
The electric power of the LO signal outputted from FET 71a is thus not the same as that of the LO signal outputted from FET 71b. Therefore, when the LO signals having different electric power values undergo opposite-phase combination in the balun circuit, the LO signals will not be cancelled, but the combined LO signal having a large electric power is outputted from the first FCPW line. Therefore, desired circuit performance cannot be achieved.